The combustion of carbonaceous fuels, such as heavy fuel oils, coals, refinery coke, and municipal and industrial waste, typically produces a plume arising from the smoke stack and can have opacity ranging from low to high. In addition, combustion of these fuels can result in the formation of slag, corrosive acids and highly carbonaceous particulate matter that alone or in combination can have a relatively negative effect on the productivity of the boilers and present a range of health and environmental risks.
The art has endeavored to solve slagging and/or corrosion problems by introducing various chemicals into the combustion system, such as magnesium oxide or hydroxide. Magnesium hydroxide has the ability to survive the hot environment of the furnace and react with the deposit-forming compounds, increasing the ash fusion temperature and/or modifying the texture of the resulting deposits. Unfortunately, the introduction of the chemicals has been very expensive due to poor utilization of the chemicals, much simply going to waste and some reacting with hot ash that would not otherwise cause a problem. U.S. Pat. Nos. 5,740,745, 5,894,806, and 7,162,960 deal with this problem, by introducing chemicals in one or more stages to directly address predicted or observed slagging and/or corrosion.
Metal-containing fuel additives are known in many forms, from homogeneous solutions in aqueous or hydrocarbon carrier media, or heterogeneous particle clusters extending all the way to visible particles formulated in the slurry form. In between is the nanoparticle range commonly defined to be metal particles above cluster size but below 100 nanometer size range. In all known instances where these metal-containing additives are used, they are introduced to the fuel/combustion/flue gas systems as single, metal-containing additive formulations or as mixtures of different metals
The current use of metals in combustion systems relies on chemistries fostered by each metal type as dictated by its unique orbital and electronic configuration acting individually. This means that in additives formulated with metal mixtures, at the time of the intended activity the metals act independently from one another during fuel combustion. In fact the physics of a combusting charge minimizes the likelihood that a mixed metal additive will land the different metal atoms within the same and/or desired and/or proper and/or preferred location on the combusting fuel species so that they may act in unison as a single entity.
The physical form of metal-containing additives of most recent interest is the nanoparticle form because of its unique surface to volume ratios and active site numbers and shapes. As is to be expected, there is interest in mixed metal nanoadditves because each metal tends to have specific functions.
Combustion systems burning hydrocarbonaceous fuels experience various degrees of combustion inefficiencies due to fuel properties, system design, air/fuel ratios, residence time of fuel/air charge in the combustion zone, and fuel/air mixing rates. These factors lead to imperfect combustion Fuel-side solutions to these problems usually involved some sort of “clean fuel” selection based upon previously determined criteria, or simply the use of additives.